The present invention relates to microwave excited electrodeless light sources and, more specifically, to an automatic starting control circuit for an electrodeless lamp powered by a solid state microwave source.
There has recently been developed a light source in which an electrodeless lamp is disposed at the ends of inner and outer conductors of a fixture in which the lamp forms a termination load for microwave power supplied at the other end of the conductors. There has also been developed various types of starting assist devices for this type of light source. The need for a starting assist is due to the high impedance mismatch between the lamp in the off state and the output impedance of the power source which results in a low percentage of the forward directed power being absorbed by the lamp. In one starting scheme, the fixture is made to be in a condition of resonance at starting to increase the power absorbed by the lamp. In another scheme, a UV light source is used to supply a flux of UV photons to the lamp. Both schemes have functioned satisfactorily in providing a starting assist. In both starting assist devices, the operator must manually disconnect the devices after the lamp is started. There exists a need for automatic connecting and decoupling of these devices if the electrodeless light source is to have enhanced versatility. It has also been found that a solid state microwave power source, when operated at its full rated power dc supply voltage, can not tolerate running into large impedance mismatches such as occur when the source is coupled to a lamp in the off state. A means of controlling this supply voltage during the lamp start-up is necessary.